Ar-Gimilzôr was the twenty-third King of Númenor and the grandfather of Ar-Pharazôn, the last King. He was a great enemy of the Faithful, and persecuted them during his reign because he was suspicious of their ties to the Elves of Tol Eressëa.

Ar-Gimilzôr completely outlawed the use of the Elven tongues once he came to power. In addition, he punished those of the Faithful who still welcomed the Elven ships that came from Eressëa, because he considered these Elves to be the "spies of the Valar", whose influence he despised. Eventually, Ar-Gimilzôr's distrust of the Faithful caused him to exile them to the east of Númenor, near the haven of Rómenna. There he kept them under surveillance.

Because of Ar-Gimilzôr's harsh treatment of them, the Faithful began leaving Númenor in increasing numbers and settling near Pelargir, their haven in Middle-earth. However, those who stayed in Númenor until its Downfall would eventually be grateful for one of Ar-Gimilzôr's policies: the chief population of the Faithful had been in the west of Númenor before the King had commanded them to move east. Because the Faithful came to the eastern side of the island before the destruction of Númenor, they were able to escape the disaster and sail to Middle-earth.

In addition to his heavy-handed policies against the Faithful, Ar-Gimilzôr completely neglected traditions long held even by the rebellious Kings before him, such as the tending of the White Tree of Númenor. The beautiful Tree, a symbol of the ancient friendship between Númenor and the West, fell into a decline. Ar-Gimilzôr was also the first of the Kings to never ascend the Meneltarma and display reverence to Eru.

Ar-Gimilzôr wed a woman "of great beauty", whose name was Inzilbêth. He did not know at the time that she herself was of the Faithful, and thus later in their marriage "there was small love between them". Inzilbêth passed on her beliefs to their elder son Inziladûn, whereas their second son Gimilkhâd was much more like his father. Ar-Gimilzôr would thus have preferred to see the Sceptre go to the hand of his younger son, but he could not change the laws of Númenor, and was succeeded by Inziladûn, who took the Quenya name of Tar-Palantir.